The present invention relates to magnetic memory systems, and more particularly to a method and system for providing a magnetic memory cell and a read/write scheme for utilizing the magnetic memory cell.
Magnetic memories are often used in storing data. One type of memory currently of interest utilizes magnetic tunneling junctions in the memory cells. A magnetic tunneling junction typically includes two ferromagnetic layers separated by a thin insulating layer. The insulating layer is thin enough to allow charge carriers to tunnel between the ferromagnetic layers. The resistance of the magnetic tunneling junction depends upon the orientation of the magnetic tunneling junctions.
FIG. 1 depicts a conventional magnetic memory cell 10 as used in a conventional magnetic memory. The conventional memory cell 10 is coupled with a voltage supply line 20 and receives a current Ir 18 during reading. The conventional memory cell 10 includes a magnetic tunneling junction 12 and a transistor 14. The magnetic tunneling junction 12 is represented by a resistor. The magnetic tunneling junction 12 is coupled to the drain of the transistor 14. The source of the transistor 14 is coupled to ground. The state of the magnetic tunneling junction 12, and thus the data stored by the conventional memory cell 10 is sensed by detecting the voltage at output 16. The output 16 is coupled to the magnetic tunneling junction 12 of the conventional memory cell 10.
FIG. 2 depicts a conventional memory array 30 using the conventional memory cell 10. The conventional array 30 is shown as including four conventional memory cells 10. The memory cells 10 are coupled to reading/writing column selection 32 via bit lines 34 and 36 and to row selection 50 via word lines 52 and 54. The bit lines are coupled to the magnetic tunneling junctions 12, while the word lines 52 and 54 are coupled to the gates of the transistors 14. Also depicted are digit lines 44 and 46 which carry current that applies a field to the appropriate conventional memory cells 10 during writing. The reading/writing column selection 32 is coupled to write current source 38 and read current source 40 which are coupled to a line 42 coupled to a supply voltage VDD 48. Also shown are current source Iw 38 and Ir 40 used in writing and reading, respectively, to the conventional memory cells 10. Also depicted are transistors 58 and 60 that are controlled using control line 62.
In order to write to the conventional memory cell 10, the write current Iw 38 is applied to the bit line 34 or 36 selected by the writing/reading column selection 32. The read current Ir 40 is not applied. Both word lines 52 and 54 are disabled. The transistors 14 in all memory cells are disabled. In addition, one of the digit lines 44 or 46 selected carries a current used to write to the selected conventional memory cell 10. The combination of the current in a digit line 44 or 46 and the current in a bit line 34 or 36 will write to the desired conventional memory cell 10. Depending upon the data written to the conventional memory cell 10, the magnetic tunneling junction will have a high resistance or a low resistance.
When reading from a conventional cell 10 in the conventional memory array 30, the write current Iw 38 is disabled and the transistors 58 and 60 are turned off by controlling the control signal through the control line 62. The read current Ir 40 is applied instead. The memory cell 10 selected to be read is determined by the row selection and column selection 32. The transistors 14 in the selected cell are on. The output voltage is read at the output line 56. For example, assuming that the resistance of the magnetic tunneling junction in a low (ferromagnetic layers polarized parallel) state is twenty kilo-ohms, that the magnetoresistance ratio is twenty percent, and that a read current used is ten micro-amps. In such a case, the output voltage would either be 240 mV or 200 mV. Thus, there is a forty millivolt difference in the signals output for different states of the conventional magnetic memory cell 10.
Although the conventional memory array 30 and the conventional memory cells 10 function, one of ordinary skill in the art will readily recognize that the difference in the signals output by the conventional memory cells 10 is relatively small. The difference in output signals between the two states of the conventional memory cell 10 is on the order of tens of millivolts. The output signals are typically on the order of a few hundred millivolts. As a result, the conventional memory cells 10 and the conventional memory array may be subject to errors.
Accordingly, what is needed is a system and method for providing a magnetic memory cell having an improved signal. The present invention addresses such a need.
The present invention provides a method and system for providing and using a magnetic memory. The method and system comprise providing at least one memory cell. In one embodiment, the at least one memory cell is arranged in an array of rows and columns. Each memory cell includes a magnetic tunneling junction and a transistor. The magnetic tunneling junction includes a first ferromagnetic layer, a second ferromagnetic layer and an insulating layer between the first ferromagnetic layer and the second ferromagnetic layer. The transistor includes a source, a drain and a gate. The source of the transistor is coupled to the magnetic tunneling junction, while the drain of the transistor is coupled with an output for reading the magnetic memory cell. A row of memory cells is selected using a row line coupled with the gate of the transistors in the row. A constant voltage is preferably provided to the selected row. All transistors"" gates in the selected row connect to the same voltage. At the same time, a column of memory cells is selected to read from using a column line coupled with the drains of the transistors in the column. In another aspect, a load is provided to the column of the array during reading. The current in the transistors, and thus the output voltage at the drain of the transistors, depends upon the state of the magnetic tunneling junction. The states of the magnetic tunneling junction can be read out by detecting the voltage at the output or by detecting the current in the load or at the drain of the transistor.
According to the system and method disclosed herein, the present invention provides a magnetic memory having a higher output signal.